Apparatus for reducing electron loading in positive-ion accelerators



Jan. 26, 1960 Filed June 30, 1958 R. J. VAN DE GRAAFF APPAR S FORREDUCING ELECTRON POSITIVE-ION ACCELERATO LOADING 2322305 RS 2Sheets-Sheet 1 R. J. VAN DE GRAAFF Jan. 26, 1960 APPARATUS FOR REDUCINGELECTRON LOADING 2,922,905

IN POSITIVE-ION ACCELERATORS 2 Sheets-Sheet 2 Filed June 30, 1958 ELECTPOA/ PA APPARATUS FOR REDUCING ELECTRON LOAD- ING- IN POSITlVE-IONACCELERATORS Robert J. Van de Graatf, Lexington, Mass., assignor to HighVoltage Engineering Corporation, Burlington, Mass, a corporation ofMassachusetts Application June 30, 1958, Serial No. 745,721

9 Claims. (Cl. 313--63) This invention relates to the acceleration ofcharged particles to high energy and in particular to apparatus forreducing electron loading in positive-ion accelerators. In accordancewith the invention, the evacuated acceleration tube of the positive-ionaccelerator is provided with a series of magnets which produce magneticfields within the acceleration tube transverse to its axis, the strengthof the magnetic fields being such that electrons are deflected away fromthe axis with only negligible deflection of the positive ions. Theinvention is useful with positiveion accelerators in which theacceleration is produced by a high potential difierence. Therefore,While in the following detailed description particular reference is madeto a belt electrostatic generator, it is to be understood that theinvention is not limited thereto but includes other high-voltageaccelerators. The invention may best be understood from the followingdetailed description there- 'of, having reference to the accompanyingdrawings in which:

Fig. 1 is a diagrammatic view in vertical central section showing apositive-ion accelerator in which the invention may be used toadvantage;

Fig. 1A is a detail showing a portion of the apparatus of Fig. 1;

Fig. 2 is a view in side elevation of the acceleration tube of theapparatus of Fig. 1 and showing one embodiment of the invention;

Fig. 3 is a section along the line 33 of Fig. 2;

Fig. 4 is a section along the line 44 of Fig. 2;

Fig. 5 is a section, but on a reduced scale, along the line 55 of Fig. 2and showing diagrammatically the trajectories of various chargedparticles;

Fig. 6 is a side View of a mount for a permanent magnet to be used inthe apparatus shown in Figs. 2 and 3;

Fig. 7 is a top view of the mount in Fig. 6;

Fig. 8 is a cross section along the line 88 of Fig. 7;

Fig. 9 is a view in longitudinal central section of a portion of amodified form of the acceleration tube of the apparatus of Fig. 1 andshowing another embodiment of the invention, wherein a permanent magnetis provided within the acceleration tube; and

Fig. 10 is a view along the line lttl1t) of Fig. 9.

Referring to the drawings and first to Fig. 1 thereof, therein is shownan electrostatic accelerator of the type disclosed in U.S. Patent No.2,252,668 to Trump. Electric charge is conveyed to a high-voltageterminal 1 by an endless insulating belt (notshown) and the electricfield produced by the accumulated charge on the terminal 1 is used toaccelerate positive ions from a positive-ion source 2 within theterminal 1 through an evacuated acceleration tube 3. The apparatus isenclosed in a tank 4 which is filled with an insulating gas underpressure. The electric field within the acceleration tube 3 not onlyaccelerates positive ions in this manner, but also accelerates towardsthe high-voltage terminal 1 any stray electrons which may be releasedwithin the acceleration tube 3. These electrons may acquire fairly highenergies and, upon striking the electrodes of the acceleration tubePatented Jan. 26, 1960 3, may release secondary electrons so that theelectron current in the tube 3 may reach considerable proportions.

This electron current constitutes a drain on the charge accumulated onthe terminal 1, which must be replenished by the charging mechanism, andtherefore reprments a load which limits the voltage attainable.Moreover, these electrons, upon striking the electrodes of theacceleration tube 3 and other objects, produce X-rays which ionize theinsulating gas. This ionization causes charge to leak off the terminal 1through the insulating gas, thus producing an additional load which mustbe supplied by the charging mechanism and which therefore further limitsthe voltage attainable. This phenomenon is known as electron loading.The invention minimizes electron loading by providing magnetic fieldswithin the acceleration tube 3 transverse to the axis of the tube. Theelectrons, which are traveling in an axial direction owing to the axialelectric field, are therefore deflected out towards the walls of thetube. In this way, the stray electrons are prevented from traveling anyconsiderable axial distance and therefore are prevented from acquiringany substantial amount of energy. Upon striking electrodes of theacceleration tube or other objects, these low-energy electrons generatemuch less X-radiation than high-energy electrons.

it is also desirable to reduce the X-rays, because they make radiationbackground in the neighborhood of the accelerator. The cumulative effectof this is undesirable for personnel and may even at time be dangerous.Also, the presence of this background interferes greatly with certaintypes of nuclear research. A test on a 6-megavolt accelerator has shownthat with the use of magnets along the tube, the X-ray background in thesurrounding region was reduced by a factor of about one hundred asmeasured with an ionization chamber. Also, measurements made with ascintillation counter showed that the maximum energy of individual X-rayphotons was reduced by a factor of about ten, rendering them far moreabsorbable by concrete walls and other material. It may also be notedthat the addition of magnets reduced the seasoning time of the tube toreach six megavolts to about four days, whereas a similar tube triedpreviously in the same generator had required several weeks ofseasoning. Thus the invention serves not only to reduce backgroundradiation but also to help prevent totalvoltage breakdown. Moreover, thebackground radiation is reduced in two ways: not only is the quantity ofradiation reduced, but the remaining radiation is very much softer andeasily stopped by absorbers.

A preferred embodiment of apparatus for producing these magnetic fieldsis shown in Figs. 2, 3 and 5. Referring thereto, each of a multiplicityof the intermediate electrodes 5 of the acceleration tube 3 is providedwith a pair of permanent magnets 6 which are magnetized in such a way asto produce a magnetic field between them transverse to the axis of thetube 3.. At each extremity of the acceleration tube 3 the magnetic fieldextends in one direction, but in the central section the magnetic fieldextends in the opposite direction. In general, the sections thus definedby the orientation of the magnetic field tend to be of increasing lengthin the direction of travel of the positive-ion beam. However, when thenumber of sections is small and is odd, the length of the last sectionmay be about the same length as that of the penultimate section. Thepurpose of this reversal of the magnetic field is to minimize the totaldeflection of the positive-ion beam. The trajectories of the chargedparticles are shown in Fig. 5. The stray electrons are released withvery low energy and relatively small mass, so that the radius ofcurvature of their trajectories is therefore relatively small so thatthe electrons are easily deflected away from the axis of the tube 3. Thepositive ions have a mass at least 1800 times that of the rest mass ofthe electrons, so that the radius of curvature of their trajectory isaccordingly very much greater, and their trajectory is almostrectilinear. Moreover, the positive ions soon acquire relatively highenergy and this energy increases throughout the length of theacceleration tube 3, so that the radius of curvature of their trajectorybecomes greater. It is in order to compensate for the varying energy ofthe positive ions that the length of each section of the accelerationtube 3 generally increases from the terminal 1 to the grounded end ofthe tube.

- The acceleration tube 3 may be of the conventional type such as thatdisclosed in the US. Patent No. 2,517,260 to Van de Graatf and Buechner,except that the intermediate electrodes 5 must have an outer diametersufliciently large to support the permanent magnets 6. In order tosupport the permanent magnets 6 a mounting arrangement such as thatshown in Figs. 6-8 may be used. Referring to Figs. 6-8, the mountingarrangement includes a block 7 of magnetic material one side of which isconcave and has a groove 8 whose width is large enough to receive one ofthe intermediate electrodes 5'. Each intermediate electrode 5 has twoholes drilled in it near its outer periphery, and each mount 7 isaflixed to the corresponding electrode 5 by means of a screw 9 whichpasses through the hole in the intermediate electrode 5. The magnet 6may simply be cemented against the fiat outer surface of the mount 7, orthe magnet 6 may be attached to the mount 7 in any other suitablemanner.

Each magnet 6 is magnetized so that the flat surface has one polarityand the rounded surface another polarity, as indicated in Fig. 3. Ofcourse, half the magnets will have their north poles at the flatsurface, and the other half will have their south poles at the flatsurface, so that a transverse magnetic field is created inside theacceleration tube, as shown by the arrows in Fig. 3. A magnetic field ofthe order of 100 gauss will usually be adequate, and is readily achievedby the foregoing construction.

It is apparent from the trajectories shown in Fig. 5 that while theelectrons are deflected so as to be intercepted by an electrode 5 afterhaving traveled usually a length corresponding to at most 3 electrodespaces, the positive ions are deflected only slightly, so that theyremain at all times close to the axis of the acceleration tube 3.Nevertheless, the magnets 6 do introduce a deviation in the position ofthe positive-ion beam which is diflicult to predict precisely.Accordingly, the invention also comprehends a novel means forcontrolling the position of the positive-ion beam which is particularlyuseful in connection with the electron-deflecting magnets 6, but whichis not limited thereto and also may be used in other situations Where itis desired to control the direction of a beam of charged particles.

Referring to Figs. 2 and 4, the positive ions emitted from the positiveion source 2 are first formed into a beam by conventional initialfocusing devices (not shown), and then the beam thus formed is directedby means of a double array of magnet pairs. Each magnet pair comprisestwo cylindrical permanent magnets 10 having rounded ends 11 and beingmagnetized uniformly throughout the axial length thereof in a directionperpendicular to said axis, as shown by the arrows in Fig. 2. One array12 of magnet pairs is arranged as an extension of theelectron-deflecting magnet system 6. This first array 12 of magnets 10therefore serves to adjust the position of the positive ion beam in theplane of Fig. 2 of the drawings. The second array 13 of magnets 10 isidentical to the first array 12, except that it is arrangedperpendicular thereto and except for the fact that fewer magnets 10 areneeded in the second array 13 because they do not have to compensate forthe deflecting action of the electron-deflecting magnets 6.

The magnets 10 of each pair are mutually connected so that rotation ofone magnet produces a similar rotation of the other magnet of the pair.The magnets 10 of the first array 12 are each connected by means of asuitable worm gear arrangement 14 to two short insulating rods 15. Themagnets 10 of the second array 13 are similarly connected to two shortinsulating rods 16. As shown in Fig. 1A, the short insulating rods 15which correspond to the first array 12 are connected by means offlexible shafts 17 and a worm gear arrangement 18 to an insulating rod19 extending from the high voltage terminal 1 of the accelerator toground. Rotation of the rod 19 by any suitable device (not shown) atground potential then produces a rotation of all the magnets 10 in thefirst array 12, and a second insulating rod 20 is used similarly torotate the magnets 10 in the second array 13. As shown by the arrows inFig. 2, all the magnets 10 in each array are arranged with theirmagnetic fields parallel to each other. When the direction of thesemagnetic fields is parallel to the axis of the acceleration tube 3, themagnetic field produced thereby within the tube 3 does not exert adeflecting force on the positive ion beam; but as the magnets 10 arerotated, a uniform component (shown by the arrows in Fig. 4) of magneticfield transverse to the axis of the acceleration tube 3 is producedwhich increases in strength as the magnets are rotated, reaching amaximum when the magnets 10 are lined up with their magnetc fieldstransverse to the axis of the acceleration tube 3.

The beam-director shown in Figs. 2 and '4 is thus a device which isadjustable from the control board of the positive-ion accelerator, sothat from the control board one can turn one knob to control theposition of the beam in one plane, and one can turn another knob tocontrol the position of the beam in the plane perpendicular thereto.

By using this beam-director it is possible to use strong fields todeflect the electrons and still be able to adjust the position of thepositive-ion beam. Without the beamdirector one cannot be sure that thenet effect is zero on the positive ions even though magnetic fieldreversal as shown in the diagram of Fig. 5 is employed. Thus thisbeam-director makes the principal idea of the invention really workableand practical.

The main advantage of the beam-director shown in Figs. 2 and 4 is thefact that it is a control feature which can operate near thehigh-voltage terminal 1 where the particles are still slow andrelatively easy to change in direction.

A second advantage of the beam-director shown in Figs. 2 and 4 is thefact that it may be positioned so that the point of curvature of thebeam is relatively distant from the grounded end of the tube 3. This isworthwhile because it is usually desirable to adjust the position of theintersection of the beam with a plane perpendicular to it near the pointof use of the beam and at the same time to make a minimum change in theangle of the intersection. For example, the beam may be introduced intoan analyzing magnet and it may be desired to adjust the position ofentrance of the beam into the magnet without changing the angle ofincidence. The method of magnetic deflection incorporated in thisbeam-director can also, of course, be used at the grounded end of thetube, although at such grounded end one can use other beamdirectingequipment, such as electrostatic plates.

The device shown in Figs. 2 and 3 is one in which permanent magnets areadded to an existing type of acceleration tube. If the aceleration tubewere built for this purpose separately, there would be many advantagesin having the permanent magnets placed inside the tube. This would makethe device very much simpler and the magnetic fields would be moreusefully applied. However, the adjustable magnets of the beam-directorare outside the acceleration tube, since it is necessary to adjust theirorientation.

Referring now to Figs. 9 and 10 of the'drawing, therein is shown amodified embodiment of the invention,

wherein the electron-deflecting magnets are placed inside theacceleration tube. The acceleration tube 3 shown in Figs. 9 and 10 maybe identical to that shown in Fig. 2, except that theelectron-deflecting magnetic field is provided by annular permanentmagnets 21 each of which is supported upon the inside periphery of anelectrode 5'. Each such permanent magnet 21 may comprise a ring ofmagnetic material which is magnetized so as to produce a transversemagnetic field within the acceleration tube 3 as shown in Fig. 10,wherein the ring 21 is shown as having two poles. On the outer peripheryof the magnet ring 21 may be inscribed a circumferential groove 22 andan electrode 5' may be shrunk into this groove 22. The magnetic field isthus concentrated in the region desired. The electrode 5 for supportingthe ring magnet 21 is identical to the other electrodes 5 of the tubeexcept that its centralaperture is larger. The inner edges 23 of eachmagnet 21 are rounded so that there is very little distortion of theelectric field within the tube introduced thereby.

Having thus described the principle of the invention together withseveral illustrative embodiments thereof, it is to be understood thatalthough specific terms are employed, they are used in a generic anddescriptive sense and not for purposes of limitation, the scope of theinvention being set forth in the following claims.

I claim:

1. Apparatus for reducing electron loading in highvoltage positive-ionaccelerators, comprising, in combination with a positive-ion acceleratorhaving an evacuated acceleration tube Within which there exists anunobstructed path along which charged particles can be accelerated toenergies in excess of one million electron volts, means for creating amagnetic field within said tube transverse to the axis thereof of astrength suflicient to deflect electrons away from said axis withoutappreciable deflection of positive ions.

2. Apparatus for reducing electron loading in highvoltage positive-ionaccelerators, comprising, in combination with a positive-ion acceleratorhaving an evacuated acceleration tube, means for creating a series ofmagnetic fields within said tube transverse to the axis thereof of astrength sufiicient to deflect electrons away from said axis withoutappreciable deflection of positive ions, said series of magnetic fieldsbeing arranged in longitudinal sequence along the tube, the orientationof said magnetic fields alternating so as to produce a focusing effecton positive ions being accelerated within said tube.

3. Apparatus for reducing electron loading in highvoltage positive-ionaccelerators, comprising, in combination with a positive-ion acceleratorhaving an evacuated acceleration tube which includes a multiplicityof'alternating non-magnetic electrodes and insulating rings, amultiplicity of pairs of permanent magnets attached to said electrodesin such a manner as to create a magnetic field within said tubetransverse to the axis thereof of a strength suflicient to deflectelectrons away from said axis without appreciable deflection of positiveions.

4. Apparatus for reducing electron loading in highvoltage positive-ionaccelerators, comprising, in combination with a positive-ion acceleratorhaving an evacuated acceleration tube which includes a multiplicity ofalternating non-magnetic electrodes and insulating rings, a series ofannular permanent magnets attached to a corresponding series of saidelectrodes and radially inwardly thereof, each of said annular permanentmagnets being magnetized so as to have poles between which a magneticfield is produced within said tube transverse to the axis thereof of astrength sufiicient to deflect electrons away from said axis withoutappreciable deflection of positive ions.

5. Apparatus for reducing electron loading in highvoltage positiveionaccelerators, comprising, in combination with a positive-ion acceleratorhaving an evacuated acceleration tube which has a source of positiveions at one end thereof, a first array of pairs of rotatable permanentmagnets flanking said tube near said source of charged particles withtheir axes mutually parallel and transverse to the axis of said tube,said magnets being magnetized all in the same direction transverse totheir respective axes, means for rotating said magnets about theirrespective axes; a second array of pairs of rotatable permanent magnetsflanking said tube near said source of charged particles with their axesmutually parallel and transverse to the axis of said tube and to theaxes of the magnets in said first array, said magnets of said secondarray being magnetized all in the same direction transverse to theirrespective axes, and means for creating a magnetic field within saidtube, in the region thereof more remote from said source of positiveions than said arrays of pairs of rotatable permanent magnets,transverse to the axis of said tube of a strength sufiicient to deflectelectrons away from said axis without appreciable deflection of positiveions.

6. The method of controlling the position of a beam of charged particlesat the exit end of an acceleration tube, which method comprises:creating a substantially uniform magnetic field component transverse tothe axis of said beam near the point of injection of said beam into saidacceleration tube, and controlling the direction of said beam by varyingthe strength and direction of said magnetic field component.

7. Apparatus for controlling the position of a beam of charged particlescomprising, in combination with an acceleration tube, a first arraycomprising at least one pair of rotatable permanent magnets flankingsaid tube with their axes mutually parallel and transverse to the axisof said tube, said magnets being magnetized all in the same directiontransverse to their respective axes, means for rotating said magnetsabout their respective axes; a second array comprising at least one pairof rotatable permanent magnets flanking said tube with their axesmutually parallel and transverse to the axis of said tube and to theaxes of the magnets in said first array, said magnets of said secondarray being magnetized all in the same direction transverse to theirrespective axes, and means for rotating said magnets in said secondarray about their respective axes.

8. Apparatus in accordance with claim 7 wherein each of said permanentmagnets comprises a solid cylinder rotatable about its axis and havingrounded ends.

9. Apparatus for controlling the position of a beam of charged particlesat comparatively near the point of use thereof without substantiallyaltering the angle of incidence of said beam at said point, comprising,in combination with an accceleration tube having a source of chargedparticles at one end thereof, a first array comprising at least one pairof rotatable permanent magnets flanking said tube near said source ofcharged particles with their axes mutually parallel and transverse tothe axis of said tube, said magnets being magnetized all in the samedirection transverse to their respective axes, means for rotating saidmagnets about their respective axes; a second array comprising at leastone pair of rotatable permanent magnets flanking said tube near saidsource of charged particles with their axes mutually parallel andtransverse to the axis of said tube and tothe axes of the magnets insaid first array, said magnets of said second array being magnetized allin the same direction transverse to their respective axes, and means forrotating said magnets in said second array-about their respective axes.

References Cited in the file of this patent UNITED STATES PATENTS2,282,401 Hansell May 12, 1942 2,417,797 Hipple Mar. 18, 1947 2,775,708Parsons et a1. Dec. 25, 1956 2,806,161 Foster Sept. 10, 1957 2,820,142Kelliher Jan. 14, 1958 2,831,996 Martina Apr. 22, 1958

